In the Grotli area an old Precambrian basement gneiss is bordered to the east by a younger supracrustal complex, assumed to be of about Eocambrian age. The three main divisions of the supracrustal complex are meta-psammites, gneisses and amphibolites mainly of volcanic origin, and micagneisses, representing sediments of graywacke type. The order of deposition within the complex is unknown. Intrusive rocks are anorthosite sills and small gabbroid !enses. The whole complex was metamorphosed and recrystallized in the amphibolite facies. Two diaphthoritic meta-eclogites of gabbroid composition are registered. A first folding (F1) broadly contemporaneous with the metamorphism, formed isoclinal, recumbent folds of all dimensions up to more than 10 km in width, with axial direction about E-W. A later folding (F2) was almost coaxial with F1, but of a concentric type. A third folding (F3) was of large amplitude and was responsible for the present structural trend of the area (mainly NNW or NE). Radioisotopic determinations gave 463 m. y. as a minimum age of the amphibolite facies metamorphism.
Geology of the Grotli area